F51
F51 (core code UNS S31803, commonly known as 2205 duplex steel) is a ferrite – austenite duplex stainless steel. With the synergistic advantages of “corrosion resistance × high strength” (its chloride corrosion resistance is superior to that of austenitic stainless steel, and its strength is twice that of austenitic stainless steel), the following is a systematic analysis from six dimensions:
Ⅰ. Standard System and Grade Codes
1. Core Executive Standards
- Forgings/Flanges: ASTM A182 (specifies high – temperature pressure – bearing forgings, such as valves and flanges, grade “F51”);
- Plates/Pipes: ASTM A240 / ASME SA – 240 (pressure vessels, heat exchanger liners/pipes);
- International Compatibility: Covers EN 10088 – 3 (Europe), JIS G4303 (Japan, SUS329J3L), etc.
2. Global Grade Correspondences
| System/Region | Grade | Description |
|---|---|---|
| U.S. UNS | UNS S31803 | Core identification (benchmark of duplex steel) |
| Chinese National Standard | 022Cr22Ni5Mo3N | Low – carbon, emphasizing corrosion resistance |
| European DIN | 1.4462 | Equivalent to F51 |
| Commercial Name | 2205 Duplex Steel | Abbreviated due to Cr≈22% and Ni≈5%, most commonly used |
Ⅱ. Chemical Composition (Mass Fraction %, ASTM Standard)
F51 constructs a duplex structure (ferrite 40 – 60% + austenite 60 – 40%) through the “four – element synergy of Cr – Ni – Mo – N”, and the roles of elements are precise:
| Element | Content Range | Core Role |
|---|---|---|
| C | ≤0.03 | Strictly control carbon to avoid intergranular corrosion (very low risk of carbide precipitation) |
| Cr | 21.0 – 23.0 | Ferrite is rich in Cr, forming a Cr₂O₃ passive film to resist uniform/pitting corrosion |
| Ni | 4.5 – 6.5 | Austenite is rich in Ni, stabilizes the structure, and improves toughness and stress corrosion resistance |
| Mo | 2.5 – 3.5 | Enhances resistance to pitting/crevice corrosion (synergizes with N, PREN≈32 – 36) |
| N | 0.08 – 0.20 | Stabilizes austenite and significantly increases strength (duplex steel strength ≈ twice that of austenite) |
| Mn | ≤2.00 | Improves workability and stabilizes the duplex proportion |
| Si | ≤1.00 | Assists in deoxidation and optimizes hot working performance |
Ⅲ. Mechanical Properties (After Solution Treatment, Room Temperature)
Due to the duplex structure strengthening, the performance of F51 far exceeds that of conventional austenitic stainless steels (such as 304/316):
| Performance Index | Typical Value (ASTM Requirement) | Comparison with 304 Stainless Steel |
|---|---|---|
| Tensile Strength | ≥620 MPa | 20% higher (304≤515 MPa) |
| Yield Strength | ≥450 MPa | 120% higher (304≈205 MPa) |
| Elongation (δ₅) | ≥25% | Slightly lower (304≤40%), still meets formability |
| Hardness | 210 – 270 HB | Higher (304≤217 HB) |
| Physical Properties | Density 7.80 g/cm³, thermal conductivity is better than austenite | Suitable for heat exchange scenarios |
Ⅳ. Heat Treatment Requirements (Activating Duplex Advantages)
- Solution Treatment (Mandatory):
- Temperature: 1020−1100∘C (hold for 1 – 2 hours to homogenize the duplex structure and dissolve carbides);
- Cooling: Rapid water quenching (inhibit the precipitation of harmful phases and retain stable ferrite + austenite);
- Role: Maximize corrosion resistance and strength, laying the foundation for processing / service.
- Welding and Post – treatment:
- Welding Materials: Select ER2209 welding wire (match the duplex composition to ensure the duplex proportion of the weld);
- Process: Control heat input (avoid ferrite coarsening). For thick – walled parts, short – term solution treatment at 1050∘C can be performed after welding (restore corrosion resistance).
Ⅴ. Main Application Fields (Strong Corrosion + High – Stress Scenarios)
Relying on “chloride corrosion resistance + high strength”, F51 dominates the following extreme environments:
- Marine Engineering:
- Seawater desalination (membrane shells, pipelines, pump valves): Resistant to pitting corrosion in 3 – 5% NaCl seawater (service life is twice that of 316L);
- Offshore platforms (mooring chains, underwater structures): Resistant to seawater + wind – wave impact, reducing cost by 30% compared with titanium alloys.
- Chemical Industry:
- Acid oil and gas fields (wellhead valves, transmission pipes): Resistant to stress corrosion cracking by H₂S + Cl⁻ (complying with NACE MR0175);
- Desulfurization and denitrification (absorption towers, spray pipes): Resistant to HCl + sulfuric acid dew point corrosion (superior to 316L in perforation resistance).
- Energy and Environmental Protection:
- Nuclear power (nuclear waste tanks, cooling pipelines): Resistant to radiation + high – temperature water corrosion, with long – term stability;
- Waste incineration (heat exchangers, flues): Resistant to short – term high temperature of 800°C + chloride corrosion.
- High – end Manufacturing:
- Food and pharmaceutical (soy sauce tanks, aseptic equipment): Resistant to organic acids + hygienic compliance (replacing 904L, reducing cost by 25%);
- Papermaking bleaching (digesters, bleaching towers): Resistant to corrosion by chlorine – containing chemicals, avoiding pulp pollution.
Key Summary
- Core Advantages:
- Corrosion Resistance: The ability to resist chloride pitting is superior to that of austenitic stainless steel (high PREN value);
- Mechanical Properties: Strength is approximately twice that of austenitic steel, and the structure can be thinned by 20 – 30%, reducing weight and saving energy;
- Cost Ratio: Performance is close to that of super duplex steel (such as F60), and the cost is 15 – 20% lower.
- Limitations:
- Long – term service at high temperatures (>300°C) requires vigilance against σ – phase embrittlement (ferrite enriches Cr, precipitating hard and brittle phases);
- Slightly higher difficulty in cold working (high strength requires greater forming force).
F51 (UNS S31803/2205) is “the king of cost – performance among duplex stainless steels”. Through the synergistic effect of ferrite + austenite, it replaces austenitic stainless steel and some nickel – based alloys in fields such as marine, chemical, and energy, balancing performance and cost, and becoming a core material for strong corrosion and high – stress scenarios.